External beam radiation therapy is a well-known treatment option available to the radiation oncology and neurosurgery communities for treating and controlling certain central nervous systems lesions, such as arteriovenous malformations, metastatic lesions, acoustic neuromas, pituitary tumors, malignant gliomas, intracranial tumors, and tumors in various parts of the body (e.g., lung, breast, prostate, pancreas, etc.). As the name implies, the procedure involves the use of external beams of radiation directed into the patient at the lesion using either a gamma unit (referred to as a Gamma Knife), a linear accelerator, or similar beam delivery apparatus. Although treating the lesions with the radiation provides the potential for curing the related disorder, the proximity of critical normal structures and surrounding normal tissue to the lesions makes external beam radiation therapy an inherently high risk procedure that can cause severe complications. Hence, the primary objective of external beam radiation therapy is the precise delivery of the desired radiation dose to the target area defining the lesion, while minimizing the radiation dose to surrounding normal tissue and critical structures.
The process of treating a patient using external beam radiation therapy consists of three main stages. First, a precise three-dimensional map of the anatomical structures in the location of interest (target volume) is constructed using any conventional three-dimensional imaging technology, such as computed tomography (CT) or magnetic resonance imaging (MRI). Second, a treatment plan is developed for delivering a predefined dose distribution to the target volume that is acceptable to the clinician. Finally, the treatment plan is executed using an accepted beam delivery apparatus.
Thus, the basic strategy of external beam radiation therapy is to utilize multiple beams of radiation from multiple directions to “cross-fire” at the target volume. In that way, radiation exposure to normal tissue is kept at relatively low levels, while the dose to the tumor cells is escalated. Thus, the main objective of the treatment planning process involves designing a beam profile, for example, a collection of beams, that delivers a necrotic dose of radiation to the tumor volume, while the aggregate dose to nearby critical structures and surrounding normal tissue is kept below established tolerance levels.
One existing method for treatment planning in external beam radiation therapy is standard manual planning. This method is referred to as forward planning because the physician solves the direct problem of determining the appropriate dose distribution given a known set of beam characteristics and beam delivery parameters. In other words, standard manual planning involves a trial-and-error approach performed by an experienced physician. The physician attempts to create a plan that is neither complex nor difficult to implement in the treatment delivery process, while approximating the desired dose distribution to the greatest extent possible. For instance, the physician may choose how many isocenters to use, as well as the location in three dimensions, the collimator size, and the weighting to be used for each isocenter. A treatment planning computer may calculate the dose distribution resulting from this preliminary plan. Prospective plans are evaluated by viewing isodose contours superimposed on anatomical images and/or with the use of quantitative tools such as cumulative dose-volume histograms (DVH's).
Standard manual planning has many disadvantages. This iterative technique of plan creation and evaluation is very cumbersome, time-consuming, and far from optimal. Thus, manual planning results in much higher costs for patients and insurers. The physician or other experienced planner can evaluate only a handful of plans before settling on one. Thus, standard planning has very limited success in improving local tumor control or reducing complications to normal tissue and critical structures, and as a result, greatly limits the quality-of-life for patients. In standard manual planning, there is no mechanism for allowing the advance imposition of clinical properties, such as, for example, an upper bound on dose received by normal tissue or the specific shape of dose-response curves to the tumor and to critical structures, on the resulting plans. Furthermore, manual planning is subjective, inconsistent, far from optimal, and only enables a small amount of treatment plans to be examined by the physician.
Another method for treatment planning in external beam radiation therapy employs computer systems to optimize the dose distributions specified by physicians based on a set of preselected variables. This approach is known as inverse planning in the medical community because the computer system is used to calculate beam delivery parameters that best approximate the predetermined dose, given a set of required doses, anatomical data on the patient's body and the target volume, and a set of preselected or fixed beam orientation parameters and beam characteristics. In order to solve the complex problem of arriving at an optimal treatment plan for the domain of possible variables, all existing methods of inverse treatment planning fix at least a subset of the set of variables. For example, a particular modality of external beam radiation therapy may include the following domain of possible variables: (1) number of beams, (2) configuration of beams, (3) beam intensity, (4) initial gantry angle, (5) end gantry angle, (6) initial couch angle, (7) end couch angles, (8) prescription dose, (9) target volume, and (10) set of target points. State of the art inverse treatment planning approaches preselect a subset of these variables and fix them during the optimization calculation.
Despite its obvious advantages over the standard manual approach, existing inverse treatment planning approaches have several disadvantages and inadequacies. As described above, these approaches do not incorporate each of the domain of possible variables into the optimization calculation. Instead, these approaches fix at least a subset of these variables to arrive at an “optimal” treatment plan. This type of “local optimization” is inherently problematic because it does not allow the full flexibility of choosing different beam geometries, beam orientation parameters, and beam parameters, imposing dose limits, and placing constraints on physical planning parameters. In other words, these approaches do not enable “global optimization” of treatment planning for external beam radiation therapy. Therefore, these approaches are limited by “less than optimal” treatment plans and, consequently, are unable to adequately control tumor growth or reduce normal tissue complications. Furthermore, there are an infinite number of possible treatment plans in inverse treatment planning, and existing methods only look at a small subset of potential plans and select the “best” from the subset. Thus, the resulting treatment plan is not a globally optimal plan.
Furthermore, existing inverse treatment planning are not well-suited for use with newer external beam radiation therapy modalities. Recent technological advances have resulted in sophisticated new devices and procedures for external beam radiation delivery, such as, for example, high-resolution multi-leaf collimators, intensity-modulated modulated radiation therapy (IMRT), and non-coplanar arc stereotactic radiosurgery (NASR). Unlike conventional radiation therapy where radiation profiles are altered via the use of a limited number of wedges, beam blocks and compensating filters, these new devices and procedures allow a large collection of beams to be shaped in any desired fashion with regard to both the geometrical shape and fluence across the field to create fixed or moving nonuniform beams of photons or charged particles. While the flexibility and precise delivery capability resulting from these advances is clearly advantageous, their full potential cannot be realized using “local optimization” schemes which do not incorporate each of the domain of possible variables into the optimization calculation, but instead fix at least a subset of these variables to arrive at an “optimal” treatment plan.
Thus, an unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.